The continuing need to consider disposing of solids wastes into subterranean earth formations has resulted in the development of several inventions for improving the quantity of wastes that may be disposed of in certain formation zones or intervals and the containment of such wastes in the chosen disposal zones. Perkins U.S. Pat. No. 5,226,749 and Perkins and Keck U.S. Pat. No. 5,314,265, both assigned to the assignee of the present invention, describe waste disposal methods wherein liquid wastes or slurried solid wastes, in particular, may be injected into subterranean earth formations from disposal wells and wherein the formations have particular physical characteristics which take advantage of the development of generally vertical two-winged hydraulic fractures extending from the disposal well and which are capable of receiving substantial quantities of waste material.
In many earth formations which may be suitable for disposal of wastes the compressive stress distribution in a selected zone which will assure containment of the waste material does permit the generation of vertical and laterally extending two-winged fractures extending from an injection well. However, the volume of waste material that may be disposed of through a particular injection well and into a particular formation zone is limited if only two opposed fracture wings are formed in a plane which is normal to the minimum in situ compressive stress and, particularly, if the formation disposal zone does not have the advantageous features described in the above-mentioned patents. Moreover, when injecting slurried solids into such an earth formation, a limiting factor in the amount of material that may be injected into the fracture is that which results from the fracture undergoing what is known in the art as "screenout". This phenomena occurs when the particulate solids carried into the fracture form a filter cake on the fracture faces and at the radial extremity or "tip" of the fracture as the carrier fluid leaks through into the formation matrix which the fracture is penetrating. Accordingly, it is desirable to contemplate the creation of multiple hydraulic fractures extending within a suitable waste disposal zone or interval. However, conventional fracturing methods and previous methods developed for disposing of liquid and slurried solids wastes have not taken into consideration a way in which multiple hydraulic fractures might be created for waste disposal purposes.
Kiel U.S. Pat. No. 3,933,205 describes a hydraulic fracturing method for fluid production wells wherein reverse flow of the fracturing fluid is carried out to effect spalling or production of formation fines into the fracture. Accordingly, the fracture is propped open and substantially filled with the spalled formation material. Kiel also contemplates that multiple fractures may extend from a vertical two-wing fracture in directions normal to the original vertical two-winged fracture since the compressive stress distribution of the formation is changed by the method of Kiel. However, this stress distribution does not extend very far from the wellbore and the secondary fractures extending from the primary two-winged fracture appear to have limited growth potential. Moreover, the method of Kiel results in the fractures being filled with formation material (fines and spalled particulates) which would not be desirable for use in connection with a method of disposing of solids waste material.
Creating multiple hydraulic fractures in subterranean earth formations for the purpose of increasing fracture volume for disposal and slurried solids wastes is considered desirable and the present invention is directed to this end.